1. Field of the Invention
This invention relates generally to semiconductor structures used to enhance optical signals. More specifically, the semiconductor structure has an active region that is finite in lateral extent. It also has a backward diode. The backward diode preferably is also finite in lateral extent and provides current confinement for the electrical current that pumps the active region.
2. Description of the Related Art
As the result of continuous advances in technology, optical devices are becoming more important and more prevalent. For example, the increasing demand for communications bandwidth has resulted in an increased interest in optical communications systems, including those that transmit data over optical fibers. This, in turn, has resulted in increased demand for optical devices for use in these systems.
One general class of optical devices is those that are based on electrically pumped semiconductor active regions. In these devices, an electrical current pumps the semiconductor active region, resulting in the emission of photons. In one common approach, different layers of material are epitaxially grown on a substrate to form the semiconductor active region and the surrounding structure. The active region can be used for different purposes, depending on the design of the rest of the device. For example, in vertical cavity surface emitting lasers (VCSELs), the active region is located within a vertical laser cavity and acts as the gain medium for the cavity. Pumping above the lasing threshold produces laser radiation that is output through one of the mirrors of the laser cavity. In edge emitting lasers, the laser cavity is oriented longitudinally. In non-lasing semiconductor optical amplifiers (SOAs), the emission of photons amplifies an optical signal passing through the active region. In vertical lasing semiconductor optical amplifiers (VLSOAs), the device has a vertical laser cavity but is used as an amplifier. The laser action within the laser cavity gain clamps the active region. A second optical signal passing through the active region experiences amplification without significant gain saturation. Other devices and uses exist for vertical laser cavities.
One drawback of these devices is they can be difficult to fabricate and operate. Because they support optical modes, they preferably should have good optical performance, such as low optical loss and efficient conversion of electrical energy to optical energy. However, because these devices are electrically pumped, they should also have good electrical performance. For example, the current path through the device preferably has low resistance in order to reduce resistive losses and preferably is channeled through the active region in order to increase the pumping efficiency. There is a need for semiconductor structures with improved optical and/or electrical performance.